The present invention relates to the cryogenic recovery of liquefied petroleum gas from a natural gas stream. In a more specific aspect, the present invention relates to a process for liquefying a natural gas stream in which the volume of liquefied petroleum gases separated from or recycled to the natural gas stream can be controlled at will and energy requirements of the process reduced.
A number of processes are known, in the prior art, for the liquefaction of natural gas, primarily to permit the practical transportation of such gases over long distances where pipelines for the transport of the gas in its gaseous state cannot be utilized. The most predominant practice is, of course, liquefaction of natural gas for transport by ocean-going vessels.
In the liquefaction of natural gas, it is customary to first remove acid gases such as CO.sub.2 and H.sub.2 S and then pass the gas through a dehydration system to remove water. Normally the gas is then cooled to a temperature sufficiently low to liquefy the same at essentially atmospheric pressure. Such cooling can be carried out by passing the gas sequentially through a plurality of cooling stages at successively lower temperatures and in which the cooling is supplied by the expansion of compressed refrigerants either derived from the natural gas itself or from an external source. One common practice is to utilize a series of successively lower boiling point refrigerants, such as propane or propylene followed by ethane or ethylene and then methane. The refrigerants utilized as cooling mediums are supplied in liquefied form by compression-refrigeration units often arranged in cascade fashion. However, the more efficient processes compress the gas to a high pressure, if it is not already at a sufficiently high pressure, prior to cooling and substitute a series of pressure reduction or flash stages for the methane cycle. This not only has the advantage of further cooling the gas as it is being reduced to essentially atmospheric pressure but gases flashed as a result of the pressure reduction steps can be utilized to further cool the liquefied gas and then by recycled to the main gas stream. While the predominant component of natural gas is methane, such gases can also contain significant amounts of C.sub.2 and higher molecular weight hydrocarbons. As the gas is progressively cooled the components of higher molecular weight than methane generally condense first. While the normally liquid components, such as C.sub.5 and higher molecular weight hydrocarbons, increase the heating value of the gas, they are of greater value as natural gas liquids for blending with motor fuels and for other purposes. In addition, failure to remove C.sub.5 and heavier hydrocarbons at an early stage can cause freezing problems in later stages of the process. It is, therefore, common practice to remove such natural gas liquids from the natural gas and recover the same as a product. This is normally done by placing one or more vapor-liquid separators at appropriate points in the cooling stream to separate the condensed C.sub.2 and higher molecular weight hydrocarbons from the main gas stream. The thus separated C.sub.2 and higher molecular weight hydrocarbons are the normally sent to another separator, which is usually a fractionating system of some type in which the C.sub.2 and higher molecular weight hydrocarbons are separated into a vapor phase stream or streams containing predominately C.sub.2 and higher molecular weight, normally gaseous, hydrocarbons and a liquid phase comprising the natural gas liquids. The vapor phase is then combined with flashed vapors from the pressure reduction steps, compressed to a pressure essentially equal to the pressure of the main gas stream, at some point upstream of the liquefaction step, and recombined with the main gas stream at such appropriate point where the pressure of the recycle gas and the main gas stream are essentially equal.
This practice of recycling C.sub.2 and normally gaseous, higher molecular weight hydrocarbons back to the main gas stream has a number of disadvantages. First of all, ethane and higher molecular weight normally gaseous, hydrocarbons are often of greater value as chemical feedstocks than as a component of the liquefied natural gas. In the case of propane and butane these components are also of greater value as separate liquefied petroleum gases or LPG. Secondly, by recombining C.sub.2 and normally gaseous, higher molecular weight hydrocarbons with flashed gases from the pressure reduction cycle the load on the compressors utilized to compress the recycle gas is significantly increased. Finally, when the C.sub.2 and normally gaseous, higher molecular weight hydrocarbons, separated from the main gas stream, are recycled directly to the main gas stream, there is not only a loss of the heat capacity of these fluids, which could be conveniently used for in-plant heating, but the energy necessary to separate individual C.sub.2, C.sub.3 and C.sub.4 hydrocarbons from such very low temperature fluids, at a later stage, is also significant.